The burning of a polymeric material may be considered as involving three steps as follows:
(1) The polymer is thermally degraded to smaller fragments.
(2) These fragments are volatilized and provide fuel for the flame.
(3) Energy is fed back to the surface of the polymer and thereby continues the formation of fuel by promoting thermal degradation of the surface and volatilization of the fragments. It is possible to retard the combustion process at any or all of these steps, and flame retardants are known that function in each of these ways.
The base polymer may itself be very resistant to thermal degradation or an additive that lessens the possibility of thermal degradation may be used. Polymers that contain a high aromatic content are known to be more resistant to thermal degradation than their aliphatic counterparts. For example, DuPont's Nomex, which is an aramid (highly aromatic polyamide) fiber made from m-phenylenediamine and isophtholyl chloride, is quite resistant. It also should be mentioned that wool is quite resistant. The polyphosphazenes are exceptionally resistant to thermal degradation. The phosphazene monomers may be used by themselves or copolymerized with another monomer to produce a polymer with the desired physical properties and thermal stability. For instance, a thermally stable polymer produced by the copolymerization of styrene and a phosphazene has been made.
An additive also might be utilized which will combine with the thermal degradation products in the condensed phase or the gas phase and render them either less volatile or less combustible. This is the main mechanism of action of the most common flame retardants.
Phosphorus containing flame retardants may function both in the condensed or in the vapor phase (as the halogens do). Elemental phosphorus has been used as a retardant for poly(ethylene terephthalate), but here the effect is mostly in the condensed phase. Both a change in the pyrolysis kinetics and an increased amount of char are observed. Red phosphorus is an effective flame retardant for oxygen containing substrates while it is only marginally useful for materials which do not contain oxygen. Triphenylphosphine oxide (Ph.sub.3 P-O), which functions exclusively in the vapor phase, also has been used as an additive for flame retardation of poly(ethylene terephthalate).
Inhibition of the energy feedback step of the combustion process may occur by increased formation of char, which functions as a thermal barrier and inhibits heat transfer. As noted above, red phosphorus in poly(ethylene terephthalate) functions partially in this way. The synergistic combination of ammonium poly-phosphate and cyanoethylated phosphine, which is an effective flame retardant system for polypropylene, owes at least part of its effectiveness as a flame retardant to the increased char formation.
Another means to reduce the thermal feedback is by producing gases by decomposition of the retardant that will cool the flame. Many of the inorganic retardants function to some extent by this process. Typical inorganic retardants are ammonium salts of carbonates, phosphates, or sulfates that cool the flame by production of NH.sub.3, CO.sub.2, SO.sub.2, etc.
The use of hydrated alumina as a flame retardant typifies the fact that a retardant does not function in only one way but rather shows some effect in many of the burning steps. Alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) competes with the substrate polymer for absorption of heat when the sample is brought into contact with an ignition source and thus slows the rate at which the temperature of the substrate polymer rises. At about 220 degrees C. the alumina trihydrate molecule decomposes via a very endothermic dehydration reaction. The evolved water vapor then reduces the decomposition of substrate by diluting and cooling the flame from combustion of the gaseous products.
The thermal decomposition of polyolefin type materials (e.g. polyethylene, polypropylene, etc.) proceeds by a random scission mechanism commencing at 290.degree. C. Since an additive must be stable at the processing temperature of 250.degree. but must undergo decomposition or reaction with the substrate at ca 300.degree., phosphorus retardants are limited to the more thermally stable oxides, acids, and phosphonium salts. Flame retardancy may be achieved by the synergistic combination of ammonium polyphosphate with either phosphine oxides, phosphonium salts and similar materials. It is known that ammonium polyphosphate alone functions in the condensed phase, presumably by combining with the degradation products and rendering them less volatile or less combustible. No char formation is observed. The phosphonium salts or oxides alone function primarily in the gas phase and presumably as radical traps. Again no char formation is observed. However, the synergistic combination is active in the condensed phase, and a large amount of char is produced. Presumably these combinations are effective by thermally shielding the polymer from the flame. Another class of phosphorus retardants which is useful for polyolefin formulations is shown below. ##STR1##
The primary decomposition pathway of poly(ethylene terphthalate) [PET] and presumably other oxygen containing polymers (polyesters, urethanes, etc.) is by a random scission of an ester linkage. Retardants that have been utilized include triphenylphosphine oxide (Ph.sub.3 PO), tris(2,3-dibromopropyl)phosphate(BrCH.sub.2 CHBrCH.sub.2 O).sub.3 PO, (commonly referred to as Tris), polymeric phosphonate and elemental (red) phosphorus. ##STR2## The first two additives are assumed to function primarily in the gas phase as radical traps. The mode of action of the above compound has not been disclosed but presumably both it and red phosphorus function in the condensed phase. As noted above red phosphorus is believed to inhibit secondary reactions, which lead to the volatile products that feed the flame, and to give rise to an increased char formation. It appears that some additives, notably red phosphorus, are quite effective for polymers containing oxygen while other classes (e.g. cyanoethyl phosphines) are effective for polymers with an all carbon backbone.
Two pathways are known for the decomposition of cellulose; these are dehydration and depolymerization. The main product of the dehydration process is a carbonaceous char along with small amounts of oxidizable materials (notably carbon monoxide), whereas the depolymerization pathway produces little or no char and all oxidizable materials. Clearly then an additive that promotes the dehydration pathway at the expense of the depolymerization pathway will be expected to provide less fuel to the flame and thus function as a flame retardant.
The treatment of cellulose with the adduct PCl.sub.3.2DMF(DMF.dbd.(CH.sub.3).sub.2 NC(O)H) results in the loss of DMF and chloride ion and the incorporation of phosphorus into the cellulose. The mode of action is probably associated with forcing the dehydration reaction since an increase in additive concentration (beyond a lower limit) increases the amount of char.
A synergistic effect is observed when an additive containing both phosphorus and nitrogen is utilized. Materials containing phosphorus linked directly to nitrogen are inherently more efficient than those not containing a direct link. The formation of phosphoramidates during pyrolysis has been suggested as a possible basis for the synergism. The ease of hydrolysis of the P-N bond and the relative inefficiency of the reaction have hampered further development of these materials.
The additives that appear to be most important for flame retardation in cellulose are the tetrakis (hydroxymethyl) phosphonium salts ((HOCH.sub.2).sub.4 P+, (referred to as THP)). Unlike the PCl.sub.3.2DMF case, there is no chemical bonding between the cellulose and the additive at normal temperature. Instead THP is polymerized with urea, and this polymer then is physically mixed with the cellulose. It is believed that the additive is thermally degraded during combustion to phosphoric and polyphosphoric acids, which then function by catalyzing dehydration.
The additives that function as flame retardants are different for each of the classes of polymers. Thus, there are certain chemical and possibly physical requirements that must be fulfilled in order for flame retardation to occur. These requirements could involve bond-making between the retardant and the substrate (as is noted with cellulose and PCl.sub.3.2DMF) or catalytic control of the reaction pathway during combustion.
The chemical changes that are involved in flame retardation may involve reaction of the retardant or a thermal degradation product of the retardant with the substrate or a thermal degradation product of the substrate. However, little is known of the basic chemistry that occurs on the surface of the burning polymer in the presence and absence of flame retardants for the simple reason that the chemical analysis of solids is difficult and requires chemical degradation of the sample.